1. Field of the Invention
The present invention relates to a semiconductor device having a photoelectric conversion element. In particular, the present invention relates to a semiconductor device having a photoelectric conversion element formed using a thin film semiconductor element and a manufacturing method thereof. Further, the present invention relates to an electronic device using the semiconductor device having the photoelectric conversion element.
2. Description of the Related Art
A large number of photoelectric conversion devices generally used for detecting electromagnetic waves are known, and for example, photoelectric conversion devices having sensitivity to ultraviolet rays to infrared rays are referred to as optical sensors (also referred to as photo sensors). Among optical sensors, an optical sensor having sensitivity to a visible light region with a wavelength of 400 to 700 nm is particularly referred to as a visible light sensor. A large number of visible light sensors are used for devices which need illuminance adjustment, on/off control, or the like depending on human living environment.
In particular, in a display device, ambient brightness of the display device is detected to adjust display luminance. This is because unnecessary power consumption can be reduced by detecting ambient brightness and obtaining appropriate display luminance. For example, such an optical sensor for adjusting luminance is used for a mobile phone or a personal computer.
In addition, not only ambient brightness but also luminance of a display device, in particular, luminance of a backlight of a liquid crystal display device is detected by an optical sensor to adjust luminance of a display screen.
In such an optical sensor, a photodiode is used for a detecting portion and output current of the photodiode is amplified in an amplifier circuit. A current mirror circuit is used for such an amplifier circuit, for example (see Reference 1: Japanese Published Patent Application No. 2005-136394).